The use of humic acid and fractions thereof as dyes is well known. ("Recent Progress in the Chemistry of Natural and Synthetic Coloring Matters," T. S. Gore et al. eds., Academic Press, N.Y. 1962, pps. 99-112.) Recently, a variety of cigarettes manufactured with brown paper wrappers have enjoyed increased popularity among smokers, and some of these cigarettes are fabricated using brown wrappers stained with humic acid. Processes for staining cigarette paper brown have included the use of dyes such as synthetic azo dyes, dyes produced from walnut shells, caramel, tannic acid and the like. However, the aforementioned stains or dyes are undesirable for a variety of reasons. First, the azo dyes may produce undesirable pyrolysis products; secondly, the amount of caramel required to produce the desired intensity of brown color considerably inhibits the burn rate of the paper. The use of such stains as tannic acid may provide the desired brown color; however, treatment of this type simultaneously reduces the porosity of the paper thereby also reducing the rate of combustion and adversely affects the gas phase composition (see Austrian Pat. No. 175,148).
Analytical studies using the humic acid treated brown wrappers have indicated in some instances a tendency toward increased gas phase constituents. Various attempts have been made to reduce constituents such as carbon monoxide, by using more efficient filter elements, by increasing the degree of ventilation in filters or by increasing the porosity of the paper or wrapper, etc.; however, none of these methods have proven to be entirely satisfactory.